EP 0 205 509 C1 (U.S. Pat. No. 4,574,653) discloses a force measuring device comprising a pot-type outer element and a piston element inserted therein by forming a narrow annular gap with the narrow annular gap and a cylindrical space formed between the inner end face of the piston element and the bottom of the pot-type outer element being filled with elastomeric material fixedly adhering to the contact surfaces and being essentially bubblefree. A pressure sensor is inserted into the bottom wall of the outer element and is in pressure transmitting contact to the elastomeric material. Thus, only forces in axial direction of the annular gap, i.e. in general supporting forces are detected.
EP 0 302 437 A2 (U.S. Pat. No. 4,864,874) discloses a force measuring device which is, for example, used in connection with a trailer coupling and has a design according to the principle of the force measuring device described above. However, no pot-type outer element is used but a cylindrical outer element and an inner element matching it and being provided with a passage. The narrow annular gap is subdivided by a cylindrical annular space in first and second annular gap regions having different annular diameters each, which annular space is formed as a transition step and is filled with elastomeric material as well. At least one pressure sensor may be embedded in the elastomeric material in the annular space. Opposing radial surfaces of the inner and outer elements form stops which narrowly limit a relative movement between the inner and outer elements in pulling direction. For measuring in the one and in the opposite directions two such force measuring devices are oppositely arranged. With such a design measurement, in only the axial direction is possible, i.e. as a pulling or pushing force. For measuring a radially directed force, i.e. a supporting force in vertical direction, a separate force measuring device is required.
Furthermore, EP 0 428 890 A2, (U.S. Pat. No. 5,060,965) in particular FIG. 8, discloses a force measuring device for detecting forces acting at an angle to each other, in particular normal to each other, where the device comprises at least two force measuring elements acting normally to each other and having a design according to the first-mentioned publication. Accordingly, there must be provided two pot-type outer elements and inner elements fitted therein, as well as a pressure sensor for each. This results in a relatively expensive manufacture of a force measuring device arranged at a common support element. Furthermore, in this publication the possibility has been disclosed to measure moments acting onto the trailer coupling, with a force measuring cell being arranged at a predetermined distance in respect of the pulling ear for measuring moments caused upon braking or by lateral forces. However, in view of the required distance from the pulling ear this design is relatively space-consuming.